The nitro carburization is a method for the surface hardening of metals, in which the metal to be hardened is enriched with carbon and nitrogen in the surface zone thereof. It has been established in the state of the art for a long time, for which reason no particular reference has to be made to prior publications here.
Usual small wall thicknesses of deep-drawn parts and stamped-bent parts in the sense of the invention are beneath 2000 μm. Such stainless steel parts are manufactured from very thin sheet metals by means of tensile compression reshaping or stamping-bending and sometimes take very filigree structures. Depending on the used method, parts having a varying or constant wall thickness can be produced, whereby these ones then entirely comprise a wall thickness of less than 2000 μm or they have such a wall thickness in at least some areas.
These filigree items are used in the most different fields of technique, such as for example as bearing covers in gearboxes, valve seats in ABS systems or as sample carriers for hazardous substances in high-precision measurements and are subject to extreme mechanical, thermal and chemical stress. The demand for corrosion resistant materials comprising a high hardness is therefore accordingly high.
The quality of such hardened items, in particular of such parts which have a high length diameter ratio (aspect ratio), has however been poor so far with respect to the mechanical resistance as well as the corrosion resistance. If established methods for surface hardening known from the state of the art are used for items having a very thin wall thickness, no industrially reproducible surface layers which meet the quality requirements can be produced.
Electron microscopic and metallographic examinations of cross sections of nitro-carburized deep-drawn articles have shown that surface layers which have been produced in this manner do not form a circumferentially closed structure. These surface layers are rather strongly fissured and comprise a plurality of channels which penetrate the surface layer and in which the environment and the unhardened core area of the deep-drawn article get into direct contact. The surface area produced in this way also comprises numerous defect sites and coarse grained particles. Due to this heterogeneous configuration, both the hardness of the surface layer and the corrosion properties thereof are subjected to such high fluctuations that an industrial application is only possible to a highly limited extend.
The reason for these results has over all to be seen in the partially extreme treatment conditions of the established methods.
Thus, DE 44 42 382 C1 shows a method for the nitro carburization of a rustproof steel article, in which the said steel article is treated in a molten alkaline bath. Such an aggressive kind of treatment causes filigree items in the sense of the invention to get partially considerable corrosion damages because of the thin wall thickness, leading to a highly inhomogeneous surface layer. Furthermore it has been proved that liquid treatments of filigree items lead to unsatisfying results because of an incomplete surface wetting.
In this connection EP 0 588 458 discloses a method for the nitration of an austenitic metal. Herein, a fluorine-based gas is applied to the metal. Fluorine-based gases are highly corrosive due to their reactivity and as a result they act aggressively on the surface of the metal. While the thus caused surface removal is even desired for articles having a high wall thickness and a corresponding high amount of material, this surface removal cannot be compensated in thin-walled deep-drawn and stamped-bent items and leads to the irreversible destruction of the surface of the article. Furthermore, the gases used there are highly toxic, highly corrosive and comprise properties that are highly endangering the environment and thus they place enormous requirements on the reactor to be used, the storage and the operational safety.